Blog

Keep up to date with the latest news

Latest Armature Reaction ( DC Machines ) MCQs – DC Machines MCQs

Latest Armature Reaction ( DC Machines ) MCQs – DC Machines MCQs

Latest DC Machines MCQs

By practicing these MCQs of Armature Reaction ( DC Machines ) MCQs – Latest Competitive MCQs , an individual for exams performs better than before. This post comprising of objective questions and answers related to Armature Reaction ( DC Machines ) Mcqs “. As wise people believe “Perfect Practice make a Man Perfect”. It is therefore practice these mcqs of DC Machines to approach the success. Tab this page to check ” Armature Reaction ( DC Machines )” for the preparation of competitive mcqs, FPSC mcqs, PPSC mcqs, SPSC mcqs, KPPSC mcqs, AJKPSC mcqs, BPSC mcqs, NTS mcqs, PTS mcqs, OTS mcqs, Atomic Energy mcqs, Pak Army mcqs, Pak Navy mcqs, CTS mcqs, ETEA mcqs and others.

DC Machines MCQs – Armature Reaction ( DC Machines ) MCQs

The most occurred mcqs of Armature Reaction ( ) in past papers. Past papers of Armature Reaction ( DC Machines ) Mcqs. Past papers of Armature Reaction ( DC Machines ) Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related Armature Reaction ( DC Machines ) Mcqs. The Important series of Armature Reaction ( DC Machines ) Mcqs are given below:

Armature Reaction -1

1. In a DC machine, the form of armature mmf waveform is __________
a) Triangular
b) Sinusoidal
c) Saw tooth
d) Rectangular
Answer: a
Explanation: All the conductors on the armature periphery between adjacent brushes carry currents (of constant value, UNcIc) in one direction and the current distribution alternates along the periphery. Because of commutator action, armature current distribution is in the steps of UNcIc. Thus, mmf waveform can be generalized by joining peak points to get triangular wave.

2. In a DC machine, the direct axis is __________
a) Axes of main poles
b) Axes perpendicular to axes of main poles
c) Not determined from the poles position
d) Can be drawn anywhere
Answer: a
Explanation: Direct axes is simply defined as the line passing through the axes of main poles. Maximum flux passes through this line. It’s also called as Direct Axis. Direct axes is always perpendicular to the geometrical neutral axis of machine.

3. In a DC machine, the form of flux density distribution (main field only) waveform is __________
a) Triangular
b) Sinusoidal
c) Saw tooth
d) Trapezoidal
Answer: d
Explanation: Flux density waveform is symmetrical and square wave with distortion at the zero points, causing wave to be in trapezoidal shape. The wave is flat topped, which get distributed due to armature mmf distribution, giving rise to the resultant flux distribution wave.

4. Due to the effect of armature reaction in DC machine, the flux per pole and generated voltage ______ and ________ respectively.
a) Increases, decreases
b) Decreases, decreases
c) Decreases, increases
d) Increases, increases
Answer: b
Explanation: The nature of armature reaction in a dc machine is cross-magnetizing with its axis (stationary) along the q-axis (at 90° elect. to the main pole axis). It causes no change in flux/pole if the iron is unsaturated but causes reduction in flux/pole (demagnetizing effect) in presence of iron saturation.

5. Armature reaction of an unsaturated DC machine is (in terms of magnetization) ________
a) Cross-magnetizing
b) Demagnetizing
c) Magnetizing
d) Cannot be determined
Answer: a
Explanation: Initially at unsaturated condition in a DC machine armature reaction lies along the q-axis. It will cause no change in flux/pole if iron is unsaturated. Now, when iron gets saturated axis gets shifted which will cause reduction in flux/pole.

6. What is the effect of demagnetizing component of armature reaction?
a) Reduces generator emf
b) Increases armature speed
c) Reduces interpole flux density
d) Results in sparking trouble
Answer: a
Explanation: When the armature of a dc machine carries current, the distributed armature winding produces its own mmf (distributed) known as armature reaction. The demagnetizing component acts in the opposite direction, reducing flux/pole in a machine, which will ultimately reduce generator emf.

7. What is the reason behind short circuit in armature?
a) Insulation failure between two commutator bars
b) Insulation failure between two turns of a coil
c) Two or more turns of the same coil getting grounded
d) Insulation failure between two commutator bars, two turns of a coil or the same coil getting grounded
Answer: d
Explanation: Armature short circuit may occur due to contact of two commutator bars or due to contact in of two coil turns as commutators are connected to respective coil sides. If two or more turns of coil are grounded then they have common end which again leads to short circuit.

8. What will happen at poles due to armature reaction in DC generator?
a) Demagnetisation of leading pole tip and magnetisation of trailing pole tip
b) Demagnetisation of trailing pole tip and magnetisation of leading pole tip
c) Demagnetising at the centre of all poles
d) Magnetising at the centre of all poles
Answer: a
Explanation: Leading pole tip (LPT) and trailing pole tip (TPT) are the two edges of the pole, they depend upon the direction of motion of the armature (in case of DC). While performing a motion the armature first saws an edge of the pole, that edge is called leading pole tip. Thus, at leading pole tip there will be demagnetization.

9. In DC generator, how armature reaction is produced?
a) Its field current
b) Armature conductors
c) Field pole winding
d) Load current in armature
Answer: d
Explanation: When the armature of a dc machine carries current i.e. load current in armature, the distributed armature winding produces its own mmf (distributed) known as armature reaction. The tothe field ampere-turns (ATf) and armature ampere-turns (ATa).

10. In a DC generator, the effect of armature reaction on the main pole flux is to _________
a) Reduce it
b) Distort it
c) Reverse it
d) Reduce and distort it
Answer: d
Explanation: When non-zero load current is passed through the armature winding, the distributed armature winding produces its own mmf known as armature reaction. According to its nature cross-magnetizing and demagnetizing, it will distort or reduce the main flux distribution.

11. In a DC machine brushes are normally located along GNA.
a) True
b) False
Answer: a
Explanation: Brushes are generally located at 900 to direct axis. The axis 900 to the direct axis is called as quadrature axis (q-axis). Generally, q-axis is along the geometric neutral axis (GNA) of machine. The brushes in a DC machine are normally located along the q-axis.

12. Armature reaction at 900 to the main field is called as ____________
a) Demagnetizing mmf
b) Cross-magnetizing mmf
c) Anti-magnetizing mmf
d) Magnetizing mmf
Answer: c
Explanation: The armature reaction flux strengthens each main pole at one end and weakens it at the other end (cross magnetizing effect). Armature reaction with axis at 90° to the main field axis is known as cross-magnetizing mmf.

13. Increase in flux density at one end of the pole is less than the decrease at the other end. This is called as ____________
a) Demagnetizing mmf
b) Cross-magnetizing mmf
c) Anti-magnetizing mmf
d) Magnetizing mmf
Answer: a
Explanation: If the main pole excitation is such that iron is in the saturated region of magnetization (this is the case in a practical machine), the increase in flux density at one end of the poles caused by armature reaction is less than the decrease at the other end, so that there is a net reduction in the flux/pole, a demagnetizing effect.

14. Peak flux density in terms of total flux density is given by _________
a) ATa (peak) = ATa (total) /P
b) ATa (peak) = ATa (total) *P
c) ATa (peak) = ATa (total) /P*P
d) ATa (peak) = ATa (total) *P2
Answer: a
Explanation: Peak flux density is defined as ratio of total flux density given to the number of poles for a given DC machine. Peak flux density in terms of total flux density is given by ATa (peak) = ATa (total) /P.

Armature Reaction – 2

1. Flux density in the interpolar region drops down because of ______
a) ATa (peak)
b) Large air gap
c) Absence of magnetic poles
d) Depends on other parameters
Answer: b
Explanation: The exact way to find the flux density owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution ATresultant(∅) = ATf(∅) + ATa(∅). The flux density of ATa(∅) which, because of large air-gap in the interpolar region, has a strong dip along the q-axis even though ATa(peak) is oriented along it.

2. Resultant ampere-turn distribution of a DC machine is given by _________
a) ATresultant(∅) = ATf (∅) – ATa(∅)
b) ATresultant(∅) = – ATf (∅) + ATa(∅)
c) ATresultant(∅) = -ATf (∅) – ATa(∅)
d) ATresultant(∅) = ATf (∅) + ATa(∅)
Answer: d
Explanation: The exact way to find the flux density owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution ATresultant(∅) = ATf (∅) + ATa(∅), where ∅ is the electrical space angle.

3. Which axis undergo shifting as a result of armature reaction?
a) GNA
b) MNA
c) Both GNA and MNA
d) Remains fixed
Answer: b
Explanation: Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current.

4. Armature reaction in a machine is demagnetizing due to _________
a) Machine is designed with iron which is slightly saturated
b) Machine is designed with iron which is unsaturated
c) Depends on the application where machine is being is used
d) Can’t tell
Answer: a
Explanation: The armature reaction in a DC machine is cross-magnetizing causing distortion in the flux density wave shape and a slight shift in MNA. It also causes demagnetization because a machine is normally designed with iron slightly saturated.

5. Which of the following are effects of armature reaction?
a) Increase in iron losses
b) Commutation problems
c) Possibility of commutator sparking
d) Increase in iron losses, commutation problems and commutator sparking
Answer: d
Explanation: Armature reaction in a DC machine is a result of distortion of main field flux distribution by armature current, which produces its own mmf called armature mmf. Directly or indirectly armature reaction is the problem occurring in DC machine as it causes various effects, which reduce machine efficiency.

 

Circuit Model, Emf And Torque MCQs

 

6. A 250 kW, 400 V, 6-pole dc generator has 720 lap wound conductors. Armature current is ____
a) 625A
b) 6.25A
c) 62.5A
d) 0.625A
Answer: a
Explanation: Armature current multiplied by the armature voltage is called as rating of a DC generator. Thus, 250 kW is the given rating while 400 V is the armature voltage. So, armature current is equal to 250*1000/400 = 625A.

7. What is the total ampere conductors/pole (in SI) if 600 lap wound conductors carry 120A current through conductors (P=4)?
a) 18000
b) 9000
c) 4500
d) 13500
Answer: a
Explanation: Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere conductors per pole is calculated by multiplying total no. of conductors with the current carried by them divided by the total no. of poles.
Ampere-conductors/pole = 600*120/4 =18000.

8. What is the total ampere turns/pole (in AT/pole) if 600 lap wound conductors carry 120A current through conductors (P=4)?
a) 18000
b) 9000
c) 4500
d) 13500
Answer: b
Explanation: Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere turns per pole is calculated by multiplying total no. of conductors with the current carried by them divided by the twice the total no. of poles.
Ampere-turns/pole = 600*120/8 =18000/2= 9000 .

9. If total ampere turns per pole is equal to 6000 A-turns, peak ampere turns for a 4-pole machine is _____
a) 24000
b) 3000
c) 1500
d) 4500
Answer: c
Explanation: Peak flux density in terms of total flux density is given by ATa (peak) = ATa (total) /P. Thus, for a 4-pole machine, ATa (total)= 6000 and P=4. Thus, Peak flux density is equal to 6000/4= 1500.

10. What is the total ampere turns per pole for 720 lap wounded conductors with carrying armature current equal to 625A in a 6-pole machine?
a) 6252 AT/pole
b) 625.2 AT/pole
c) 62.52 AT/pole
d) 8252 AT/pole
Answer: a
Explanation: For a given machine number of parallel paths is equal to 6. So, conductor current will be equal to armature current divide by no. of parallel paths i.e. 625/6. Conductor current = 104.2 A. Total armature ampere-turns, ATa = ½(720*104.2/6)= 6252 AT/pole.

11. For 6252 AT/Poles, if brush shift is of 2.50 mech. Degrees, what will be the demagnetizing ampere-turns per pole for a 6-pole DC machine?
a) 521
b) 5731
c) 5231
d) 571
Answer: a
Explanation: From given mech. Degrees shift we need to find electrical degrees shift. Electrical shift= mechanical shift*(P/2). Thus, electrical shift is equal to 7.50. Demagnetizing ampere-turns is given by 6250*(2*7.5/180) = 521 AT/Pole.

12. For 6252 AT/Poles, if brush shift is of 2.50 mech. Degrees, what will be the cross-magnetizing ampere-turns per pole for a 6-pole DC machine?
a) 521
b) 5731
c) 5231
d) 571
Answer: b
Explanation: For calculations, from given mech. Degrees shift we need to find electrical degrees shift. Electrical shift= mechanical shift*(P/2). Thus, electrical shift is equal to 7.50. Cross-magnetizing ampere-turns is given by 6250*(1-2*7.5/180) = 5731 AT/Pole.

Ways to Reduce Effects of Armature Reaction – 1

1. The armature reaction in a DC machine causes distortion in the main field flux. Effect of armature reaction can be reduced by_________
a) Increasing the length of air gap
b) Decreasing the length of air gap
c) Increasing the number of poles
d) Decreasing the number of poles
Answer: b
Explanation: Decreasing the air gap is simple to say but hard to achieve, due to various other limitations. But it is one of the way to reduce dropping down of armature mmf flux density distribution, which further reduces the distortion of resultant flux density.

2. In order to neutralize armature mmf perfectly under the pole shoe, the ampere-conductors of compensating winding must be (pole arc/pole pitch=1) _______
a) Not equal to the total armature ampere conductors under the pole shoe
b) Equal to the total armature ampere conductors under the pole shoe
c) Half of the total armature ampere conductors under the pole shoe
d) Twice of the total armature ampere conductors under the pole shoe
Answer: b
Explanation: The number of ampere-turns required for compensating winding is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)]*(pole arc/pole pitch). Here, pole arc= pole pitch, is given. Thus ampere-conductors required are equal to the total armature ampere conductors under the pole shoe.

3. What is the ampere turns per pole for compensating winding in DC machines?
a) (pole arc / pole pitch) * armature ampere turns per pole
b) (pole pitch / pole arc) * armature ampere turns per pole
c) (pole arc / pole pitch) * total ampere conductors per pole
d) Cannot be determined
Answer: a
Explanation: The number of ampere-turns required for this purpose is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)]*(pole arc/pole pitch). The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization.

4. Where are no equalizer rings connected?
a) Only wave winding
b) Only lap winding
c) Both wave winding and lap winding
d) Cannot be determined
Answer: a
Explanation: The armature coils forming each of the two parallel paths are under the influence of all pole-pairs so that the effect of the magnetic circuit asymmetry is equally present in both the parallel paths resulting in equal parallel-path voltages. Thus, equalizer rings are not needed in a wave winding.

5. Inequality in brush arm currents caused due to different emf induced in different parallel paths may give rise to copper losses. These effects can be avoided by using _________
a) Compensating windings
b) Interpoles
c) Equalizer rings
d) Cannot be determined
Answer: c
Explanation: The armature coil in lap windings have equal to or more than 2 parallel paths. Under the influence of pole pairs there may arise asymmetry in various paths resulting in different parallel path voltages. So, equalizer rings are connected in order to achieve symmetry again.

6. Which of the following method will completely neutralize the armature reaction in a DC machine?
a) Only compensating winding
b) Only interpoles
c) Compensating winding and interpoles
d) Cannot be determined
Answer: c
Explanation: To neutralize the cross magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots of the pole faces. Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine.

7. What is the exact location of compensating winding?
a) Across armature
b) In series with armature
c) Across armature or in series
d) Cannot be determined
Answer: b
Explanation: By adding a compensating winding in the pole face plate which carries armature current in the opposite direction of current in the adjacent armature windings, the position of the flux at the pole face plate can be restored to the position it would have with zero armature current.

8. Exact purpose of compensating winding in a DC generator is _________
a) Mainly to reduce the eddy currents by providing local short-circuits
b) To provide path for the circulation of cooling air
c) To neutralise the cross-magnetising effect of the armature reaction
d) Cannot be determined
Answer: c
Explanation: Armature reaction can be reduced with the help of compensating winding. To neutralize the cross-magnetizing effect of armature reaction in a DC machine, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots of the pole faces.

9. Function of interpole flux is_________
a) Neutralise the commutating self-induced emf
b) Neutralise the armature reaction flux
c) Neutralise both the armature reaction flux as well as commutating emf induced in the coil
d) Perform none of the above functions
Answer: c
Explanation: Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine i.e. to neutralise both the armature reaction flux as well as commutating emf induced in the coil.

10. Why dummy coils are connected in DC machine?
a) To reduce eddy current losses
b) To enhance flux density
c) To amplify voltage
d) To provide mechanical balance for the rotor
Answer: d
Explanation: Dummy coils are connected in armature winding of type wave winding. In a lap winding Yc= +/-1 irrespective of the number of armature coils so that coils can always be chosen to completely fill all the slots (C =1/2 US). In a wave winding the number of coils must fulfil the condition C =P/2 Yc +- 1 while at the same time C must also be governed by C =1/2 US.

11. The most likely cause(s) of sparking at the brushes in a DC machine is /are ____________
a) Open coil in the armature
b) Defective interpoles
c) Incorrect brush spring pressure
d) Open coil in armature, defective interpoles and incorrect brush spring pressure
Answer: d
Explanation: Brushes are the point of contacts between rotating and non-rotating parts. So, if point of contact is rough then brushes will face damage and will lead to sparking in them. Defective interpoles will contribute in irregular commutation ultimately it will lead to sparking.

12. Each of the following statements regarding interpoles is true except______________________
a) They are small yoke-fixed poles spaced in between the main poles
b) They are connected in parallel with the armature so that they carry part of the armature current
c) Their polarity, in the case of generators is the same as that of the main pole ahead
d) They automatically neutralize not only reactance voltage but cross-magnetisation as well
Answer: b
Explanation: Interpoles are located in interpolar regions hence, they are called as interpoles. They are small narrow poles which speed up the commutation process (Also called commutating poles/ compoles).

Ways to Reduce Effects of Armature Reaction – 2

1. Axis undergo shifting as a result of armature reaction, can be balanced by ______
a) Increase in armature current
b) Decrease in armature current
c) Introducing interpoles
d) Removing interpoles
Answer: c
Explanation: Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current, due to armature reaction. This effect is countered by the interpoles placed in GNA.

2. The choice of average coil voltage determines the minimum number of commutator segments for its design.
a) True
b) False
Answer: a
Explanation: The maximum allowable voltage between adjacent segments is 30–40 V, limiting the average voltage between them to much less than this figure. The choice of the average coil voltage determines the minimum number of commutator segments for its design, to avoid any flashover and ultimately short circuit.

3. Compensating winding will provide incomplete neutralization ____________
a) Under pole region
b) In interpolar region
c) Everywhere
d) Complete neutralization
Answer: b
Explanation: The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization. Further, the effect of the resultant armature mmf in interpolar region is rendered insignificant because of large interpolar gap.

4. Cross-magnetizing effect of armature reaction can be reduced by __________
a) Removing saturation in teeth and pole-shoe
b) Making smooth pole shoes
c) Introducing saturation in teeth and pole-shoe
d) Cannot be determined
Answer: c
Explanation: The cross-magnetizing effect of the armature reaction can be reduced by making the main field ampere-turns larger compared to the armature ampere-turns such that the main field mmf exerts predominant control over the air-gap flux. This is achieved by introducing saturation in the teeth and pole-shoe.

5. Cross-magnetizing effect of armature reaction can be reduced by __________
a) Removing saturation in teeth and pole-shoe
b) Making smooth pole shoes
c) Chamfering the pole shoes
d) Cannot be determined
Answer: c
Explanation: By chamfering the pole-shoes which increases the air-gap at the pole tips. This method increases the reluctance to the path of main flux in a DC machine but its influence on the cross-flux is much greater. This is because the cross flux has to cross the air-gap twice.

6. To counter the effect of shift in MNA due to armature reaction, which of the following component can be shifted?
a) Poles
b) Commutator
c) Brushes
d) Cannot be determined
Answer: c
Explanation: To counter the effect of shift in MNA due to armature reaction, the brushes could be shifted. A small brush shift in appropriate direction, in the direction of rotation for generator and in opposite direction for motor, also helps in commutation.

7. Calculate the number of conductors on each pole piece required in a compensating winding for a 6-pole lap-wound dc armature containing 286 conductors. The compensating winding carries full armature current. Assume ratio of pole arc/ pole pitch = 0.7.
a) 6
b) 8
c) 9
d) 7
Answer: a
Explanation: The number of ampere-turns required for compensating winding is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch).
Ncw/pole = (Z/2AP) * (pole arc/pole pitch) = [286 / (2*6*6)] 0.7 = 2.78.
Compensating conductors/pole = 2 * 2.78 = 6 (nearest integer).

8. A compensating winding with ampere-turns greater than peak ampere turns is required in order to neutralize the effect of armature reaction because _____________
a) Pole arc = Pole pitch
b) Pole arc > Pole pitch
c) Pole arc < Pole pitch
d) Can’t be determined using pole arc, pole pitch
Answer: b
Explanation: The number of ampere-turns required for compensating winding in a DC machine is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are more then, pole arc is definitely greater than pole pitch.

9. If pole arc is less than pole pitch, a compensating winding will have ampere-turns _________ (compare to peak ampere turns).
a) Less
b) Equal
c) More
d) Can’t be specified
Answer: a
Explanation: The ampere-turns required for compensating winding in a DC machine is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are less then, pole arc is smaller than pole pitch and vice-versa.

10. What is the pole arc/pitch ratio, if 360 AT compensating winding is used where 1960AT is peak value?
a) 0.7
b) 0.8
c) 0.9
d) 0.6
Answer: d
Explanation: The ampere-turns required for compensating winding in a DC machine is ATcw /pole = ATa (peak) *(pole arc/pole pitch). If compensating winding of 360AT is used the, 360/1960 will give ratio of pole pitch /pole arc, equal to 0.6.

11. Only drawback of compensating winding is _______
a) Cost
b) Unavailability of material
c) Construction
d) Not a single drawback
Answer: a
Explanation: Compensating winding is the best method in order to prevent the effect of armature reaction and its consequences. Only problem is compensating winding is expensive, but it is must to use them in machines with heavy overloads occur.

Latest Armature Reaction ( DC Machines ) MCQs – DC Machines MCQs